This invention relates to chemical sensors based on surface plasmon resonance.
Optical surface plasmon devices are effective chemical detectors because of the extreme sensitivity of the plasmon resonance to environmental changes. Surface plasmon devices are used in numerous sensing applications including pollution monitoring, toxic gas detection, biochemical measurements and biomedical monitoring. Surface plasmon resonance sensors respond to changes in refractive index or thickness of films adsorbed to the sensing layer and are of particular interest in detecting immunological reactions. Besides sensor applications, the surface plasmon devices have also proven effective as polarizers and as polarization splitters.
A surface plasmon wave is a lossy TM-polarized electromagnetic wave supported by a metal-dielectric interface. When the metal has a finite thickness, individual surface plasmons are supported on both sides of the metal. Further, if the metal is sufficiently thin, such as on the order of the penetration depth of the optical wave, the two surface plasmons are coupled, forming the symmetric and antisymmetric bound and leaky modes.
Existing surface plasmon sensors are classical attenuated total reflection devices. Excitation of the surface plasmon modes is monitored by measuring reflected power as a function of angle. However, those sensors may be costly, are difficult to use for simultaneous multiple analytical evaluations, are not amenable to chemical analysis in restricted geometries and are difficult to apply in remote sensing.
Recently, efforts have shifted toward designing surface plasmon sensors based on optical waveguides coated with thin metallic layers. In those configurations, analyte detection is afforded by monitoring the ratio of the TM and TE modal insertion losses. That ratio is referred to as the extinction ratio. Surface plasmon waves are excited by TM-polarized waveguide modes. Excitation occurs when the surface plasmon waves and the TM waveguide modes are phase matched, resulting in a coupled wave system. The resulting normal modes have complex propagation constants, owing to the loss induced by the surface plasmon. TE-polarized waveguide modes do not interact with the surface plasmons and experience a small, relatively constant loss due to the presence of the metal layer. Effectively, in the TM polarization, the metal-clad waveguide functions as an asymmetric directional coupler in which a first arm is the metal layer supporting the plasmon modes and a second arm is the dielectric waveguide. Surface plasmon excitation is strongly dependent on wavelength. When the surface plasmon mode and the waveguide mode are phase-matched, the resulting TM normal mode propagates with high loss. When the two modes are not phase-matched, propagation loss is considerably lower. Typically, a thin dielectric buffer layer is inserted between the waveguide and the metal layer to control insertion losses. In most surface plasmon waveguide designs, that buffer layer is very thin, giving rise to strong coupling between the surface plasmons and the waveguide modes. That strong coupling makes analysis by traditional coupled mode techniques inappropriate. Additionally, the thicknesses of both the metal layers and the buffer layers in existing sensors must be adjusted to allow plasmon resonance to occur at the proper excitation wavelength. That complicates the design process.
Needs exist for highly sensitive surface plasmon waveguide sensors that are precise, efficient, suitable for diverse sensing applications and capable of remote operation.